1. Field of the Invention
The invention relates in general to a pixel, and more particularly to an image sensor pixel and a driving method thereof.
2. Description of the Related Art
Referring to FIG. 1, a schematic diagram of first conventional image sensor pixel is shown. The conventional image sensor pixel 15a is for sensing an X-ray image. The conventional image sensor pixel 15a comprises a photodiode PD1 and a transistor T. The photodiode PD1, which can be realized by such as a PIN photodiode, comprises a photodiode body D and an equivalent capacitor Cp which are connected in parallel. An anode of the photodiode PD1 is coupled to a bias line 16 which is used for transmitting a bias voltage to an anode of the photodiode PD1, so that the photodiode PD1 is operated in a reverse bias state. A control terminal of the transistor T is coupled to a gate line 13. A first terminal of the transistor T is coupled to a cathode of the photodiode PD1, and a second terminal of the transistor T is coupled to a data line 14.
The transistor T can be realized by such as is thin film transistor (TFT), and the control terminal of the transistor T can be realized by such as a gate. If the current flows to the second terminal of the transistor T from the first terminal of the transistor T, the first terminal and the second terminal of the transistor T are respectively realized by such as a drain or source. To the contrary, if the current flows to the first terminal of the transistor T from the second terminal of the transistor T, the first terminal and the second terminal of the transistor T are respectively realized by such as a source or a drain.
The X-ray is converted into a visible light by a fluorescent layer, then the visible light illuminates the photodiode PD1. After the photodiode PD1 is illuminated by the visible light, the covalent bond is broken and electron hole pairs are generated. The holes move towards the bias line 16, and the electrons are stored in the equivalent capacitor Cp. Then, the electrons stored in the equivalent capacitor Cp are read by the data driving circuit so as to obtain a corresponding image signal.
The capacitance of the equivalent capacitor Cp of the photodiode PD1 equals εA/d, wherein d denotes the distance between an upper electrode and a lower electrode of the equivalent capacitor Cp, A denotes the overlapping area between an upper electrode and a lower electrode of the equivalent capacitor Cp, εdenotes a corresponding dielectric coefficient of the equivalent capacitor Cp. The capacitance of the equivalent capacitor Cp of the photodiode PD1 is subjected to the optical conversion efficiency of the photodiode PD1, and the larger the overlapping area A, the better the optical conversion efficiency. Thus, the capacitance of the equivalent capacitor Cp shall not be too small otherwise the number of electrons outputted by the equivalent capacitor Cp when read by the data driving circuit will be too small. To assure that the number of electrons outputted by the equivalent capacitor Cp is sufficient, the capacitance of the equivalent capacitor Cp must be designed to be large enough. To summarize, the bottleneck of the resistive-capacitive delay (RC Delay) lies in the equivalent capacitor Cp. The resistance of the RC delay refers to the on-state resistance of the transistor T, and the capacitance of the RC delay refers to the equivalent capacitor Cp.
Referring to FIG. 2, a schematic diagram of second conventional image sensor pixel is shown. The conventional image sensor pixel 15b comprises a photodiode PD2 and a transistor T. The photodiode PD2, which can be realized by such as a MIS photodiode, comprises a photodiode body D and an equivalent capacitor C which are connected in serial. A cathode of the photodiode PD2 is coupled to a bias line 16 used for transmitting a bias voltage to a cathode of the photodiode PD2, so that the photodiode PD2 is operated in a reverse bias state. A control terminal of the transistor T is coupled to the gate line 13. A first terminal of the transistor T is coupled to an anode of the photodiode PD2, and a second terminal of the transistor T is coupled to a data line 14. Similarly, the capacitance of the equivalent capacitor C must be designed to be large enough. Thus, the bottleneck of the RC delay lies in the equivalent capacitor C.
To sense a dynamic object, at least 30 frames must be read per second. For the conventional image sensor pixel, the equivalent capacitor Cp or the equivalent capacitor C has a larger capacitance, so the read speed is subjected to the restriction in the RC Delay of the conventional image sensor pixel and the conventional image sensor pixel is thus inapplicable to sensing a dynamic object.